1. Field of the Invention
The present invention relates to a solution casting process and cellulose ester film. More particularly, the present invention relates to a solution casting process capable of producing a polymer film with high optical performance, and cellulose ester film produced by the process.
2. Description Related to the Prior Art
Cellulose acylates are used as a support of polymer film contained in photosensitive materials, such as photographic films, owing to advantageous characteristics, for example rigidity, non-flammability, and the like. A typical example of cellulose acylate is cellulose triacetate (TAC) having an average acetylation degree of 57.5-62.5%. Also, the polymer film of the cellulose triacetate (TAC) is used as a protection film of a polarizer, or an optical compensation film (view angle enlarging film or the like), any of those being incorporated in a liquid crystal display (LCD) panel. This is effective because of optically utilizing the highly isotropic property of the polymer film.
A solution casting is a widely used method of producing polymer film of the cellulose triacetate (TAC). The solution casting is capable of producing a base film of photosensitive material or transparent sheet on a liquid crystal display panel. A process of producing polymer film of cellulose triacetate (TAC) is typically disclosed in JIII Journal of Technical Disclosure Monthly (Japan Hatsumei Kyokai, Kokai Giho), No. 2001-1745. At first, cellulose triacetate (TAC) is dissolved in a mixed solvent of which a main component is dichloro methane, to prepare dope or polymer solution. After this, the dope is cast on a support, for example a band-shaped support or rotatable drum-shaped support, to form cast film. When the cast film dries to have a self-supporting property, a self-supporting cast film is stripped by a stripping roller. The cast film is sufficiently dried and cooled, and wound as a roll of the polymer film.
U.S. Pat. No. 6,503,581 (corresponding to JP-A 2001-188128) discloses the use of a different from the cellulose triacetate (TAC) for the purpose of enlarging durability against moist and heat. According to the patent document, cellulose acetate propionate (CAP) is produced by acylation of an acetyl group (—CO—CH3) and acylation of a propionyl group (—CO—C2H5). The cellulose acetate propionate (CAP) is used as one raw material of the polymer film. The cellulose acetate propionate (CAP) contains a longer acyl group at side chains than that of the cellulose triacetate (TAC), and characteristically has a higher lipophilic property and a lower hydrophilic property. Solubility of the cellulose acetate propionate (CAP) in organic solvents can be higher. Preparation of a dope from the cellulose acetate propionate (CAP) can be easier.
The polymer film formed from the cellulose acetate propionate (CAP) has an advantage in high durability against moist and heat because of a hydrophobic characteristic of a side chain of the acyl group contained in the cellulose acetate propionate (CAP). Also, the cellulose acetate propionate (CAP) in the polymer film contains a propionyl group. Orientation of molecules of the cellulose acetate propionate (CAP) can easily occur because of readiness in exertion of intermolecular force between its molecules. Thus, a high value of retardation Re in the in-plane direction of the polymer film can be obtained by the cellulose acetate propionate (CAP). A high value of retardation Rth in the thickness direction of the polymer film can be obtained. The cellulose acetate propionate (CAP) is effectively used as optical film in the liquid crystal display (LCD) panel of a vertical alignment (VA) type in which optical compensation is required.
There is a technique of reducing a value of a cell gap which is an interval between base plates of glass for squeezing molecules of the liquid crystal. This is for the purpose of quick response in the liquid crystal display (LCD) panel. A high value of the retardation Re in the in-plane direction can be obtained and is advantageous in view of the optical compensation. However, the retardation Rth in the thickness direction will be considerably high in the manner of the retardation Re in the in-plane direction, and too high in consideration of the cell gap, to result in lowering a quality of imaging of the liquid crystal display (LCD) panel. The casting according to the above patent document is conditioned in the same manner as when the cellulose triacetate (TAC) is used as a raw material. There remains a problem in too high values of both of the retardation Re in the in-plane direction and the retardation Rth in the thickness direction.
In considering the disclosure of U.S. Pat. No. 6,503,581 (corresponding to JP-A 2001-188128), the number of carbon atoms contained in a substitution group of acylation can be higher. Conceivable examples of such acyl groups are butyryl (butanoyl) group (—CO—C3H7), pentanoyl group (—CO—C4H9) and hexanoyl group (—CO—C5H11). Those examples have a longer side chain than that of the acetyl group, so that tendency of occurrence of intermolecular force of the polymeric molecules will be relatively high. The polymer film, if produced from raw material containing cellulose esters having the substitution group of any of those, will have high orientation. The polymer film will be suitable as a phase difference film, because of highness of the retardation Re in the in-plane direction and the retardation Rth in the thickness direction. However, a serious problem may arise in difficulty in the casting due to differences in the characteristics from the cellulose triacetate (TAC), as a result of an experiment under the same condition as the cellulose triacetate (TAC). Also, the value of the retardation Rth in the thickness direction may be too high, so the use of the polymer film is considerably difficult as a phase difference film.